Go Back a page in the tutorial and I think you'll find that you've skipped a step. Where you have the black line which is the outer edge of your solid, you now need to select 'Offset Entities' and then set a distance (as it states on this page shown) of 2mm. You should notice a yellow line appear that is the same shape as your black line. Now, if the yellow line is on the outside of the black line box then do as the instruction says and click the arrow next to your offset parameter so that the yellow lined box now appears inside your black line box and you'll see that the yellow lined box is as per the diagram they show in the tutorial.